Pressure-balanced (PB) control valves require a dynamic seal between the PB plug head and the sleeve in which the plug slides up and down. The pressure contained on the top of the plug head is nearly equal to the pressure on the bottom of the plug head, conventionally via holes that pass through the plug head. This “balance” effect on the plug head allows the plug head to be moved through high valve pressure drops with minimal actuator load. This reduces the overall cost of the valve since smaller actuators can be used to control the flow through the valve.
A dynamic seal is conventionally used to seal the area between the fluid flow path and the chamber on the top of the plug head. The dynamic seal is most often located on the outer diameter of the plug head. In cryogenic applications (−196° C.), the plastic, spring energized seals that are generally used shrink at a rate many times that of the metal parts around it. As a result the seal will leak when the plug is on the seat (valve is shut). Some cryogenic applications require leakage levels to be very low when the valve is shut. Outer diameter seals by themselves are not capable of getting very low levels of leakage.